1. Field of the Invention
The present invention relates to a sensor circuit for a telephone line. More specifically, the present invention relates to a sensor circuit for a telephone line which is provided in a network control unit (NCU).
2. Description of the Prior Art
A circuit diagram of a conventional example of an NCU is shown in FIG. 5. The NCU 10 includes terminals 12 and 14 respectively connected to an intra-office switch (not shown). Relay switches 16 and 18 have movable contacts 16a and 18a and individual contacts 16b and 16c and 18b and 18c, respectively. The movable contact 16a is connected to the terminal 12 via a telephone line 20, and the movable contact 18a is connected to the terminal 14 via a telephone line 22 in which a line current supervisor circuit 24 is inserted. The individual contacts 16b and 18b are connected to both ends of a primary winding 26a of a line transformer 26, respectively. The line transformer 26 includes a secondary winding 26b.
In addition, the individual contacts 16c and 18c are connected to a tone detection circuit 28 which is composed of a capacitor 30 and a line transformer 32. The line transformer 32 includes a primary winding 32a and a secondary winding 32b coupled to the primary winding 32a.
The secondary winding 26b of the 26 is connected to a two-line/four-line (2W/4W) conversion circuit 34 which converts two lines of the telephone line into four lines which are connected to a signal processing circuit 36 incorporated in another electronic device, for example, a facsimile machine. The secondary winding 32b of the line transformer 32 is also connected to the signal processing circuit 36 in the facsimile machine.
The signal processing circuit 36 determines whether an electrical signal flowing in the telephone lines 20 and 22 is a signal for a telephone (not shown) connected to terminals 38 and 40 or a signal for the facsimile machine. When it is determined that the electrical signal is for the telephone, the signal processing circuit 36 controls the relay switches 16 and 18 so as to connect the movable contacts 16a and 18a to the individual contacts 16c and 18c, respectively and, when it is determined that the electrical signal is for the facsimile machine, the movable contacts 16a and 18a are connected to the individual contacts 16b and 18b, respectively by the signal processing circuit 36.
The line current supervisor circuit 24 is inserted in the telephone line 22, as described above. The line current supervisor circuit 24 includes a pair of photocouplers 42 and 44. Photocoupler 42 is composed of a light emitting element 42a which emits a light signal when the line current flows in the direction of arrow A and a light receiving element 42b which receives the light signal emitted by the light emitting element 42a. Photocoupler 44 is composed of a light emitting element 44a which emits a light signal when the line current flows in the direction of arrow B and a light receiving element 44b which receives the light signal emitted from the light emitting element 44a. Signals from the light receiving elements 42b and 44b are output at terminals 46 and 48, respectively, and the terminals 46 and 48 are connected to a detection circuit (not shown) for detecting an on state or an off state of a hook switch (not shown) of the telephone and the signal processing circuit 36.
Then, if the hook switch is turned off by releasing a hand of the telephone set, the line current flows in the direction of arrow A, and therefore, the light emitting element 42a emits a light signal which is then received by the light receiving element 42b. Therefore, an output signal is applied to the signal processing circuit 36 via the terminal 46. On the other hand, if the line current flows in the direction of arrow B, the light emitting element 44a emits a light signal which is then received by the light receiving element 44b. Therefore, an output signal is applied to the signal processing circuit 36 via the terminal 48.
As described above, the conventional example of the NCU shown in FIG. 5 includes the line current supervisor circuit 24 and a separate tone detection circuit 28. The tone detection circuit 28 includes the line transformer 32 which is large and expensive. The line current supervisor circuit 24 is constructed by the photocouplers 42 and 44, as described above. Therefore, if the line current supervisor circuit 24 and the tone detection circuit 16 are individually mounted on a circuit board, a substantially large space is required for mounting these circuits, and therefore, the NCU itself becomes large and the cost thereof also becomes high.